I've unexpectedly noticed while restoring an old HP-16C that the aluminium zone around the display is not really just aluminium; there seems to be some very thin film over it, maybe to give it a more mate finish, maybe for a bit of protection against scratching, maybe both.

I wonder if other members know what it is and how can it be re-applied to those unfortunate units that where not well taken care of and are full of scratches.

And one other thing. When polishing the unit aluminium parts, in those cases where they are heavily scratched, I managed to get a really good result, but still, I can't get the corners correct no matter how much I try, it just seems impossible to get to them and make them look exactly like the rest of the bezel. Have any tips for that?